Sound consists of alternating regions of compression and rarefaction and is transmitted directionally as a pressure wave. Olson, in U.S. Pat. No. 1,885,001 describes what has come to be known as a “ribbon microphone” for converting sound-driven motion of a thin strip of conductive material in a magnetic field into an electrical potential that can be amplified and has high fidelity in following the incident sound wave. Motion of the conductive ribbon cuts flux lines in a magnetic field and thus generates a potential in the metal. The microphone is termed “electrodynamic” because the potential matches the velocity of the ribbon in the magnetic field. The ribbon as disclosed by Olson is crafted to be displaced by the impact of the pressure wave and has little or no elastic (i.e., “restorative”) modulus. Typically aluminum is used, and the ribbon is transversely corrugated to reduce any stiffness. Because of its fidelity at a broad mid-range of frequencies, microphones of this type were instrumental in growing the popularity of the early broadcasting industry and in manufacture of high quality reproductions of sound recordings. These microphones are experiencing renewed appreciation for their technical qualities and improvements continue to be made.
Duncan, in U.S. Pat. No. 2,552,311 offers a solution to the problem of a loss of response at higher frequencies. (As noted by Olson, the shorter the acoustic circumference around a “baffle” or pole piece, the greater the high frequency range will be; a larger pole piece will result in a lower high frequency roll off; high frequency roll-off (i.e., any lack of response to higher frequencies) is due to the circumference of the pole piece being longer than the peak-to-peak distance of a sound wave in air, which grows shorter as the frequency increases.)
Duncan teaches magnetic “pole shoes” that are shaped to taper along the length of the ribbon so as to increase the microphone response at higher frequencies. However, the frequency response is surprisingly irregular. A related approach is developed in Fisher, U.S. Pat. No. 3,435,143, who tapers not only the magnetic pole pieces, but also the ribbon. While narrowing the ribbon is disadvantageous because it increases electrical resistance, Fisher taught that the shape of the magnetic pole pieces could be tapered almost to a point so as to solve the path length dilemma. Royer, in U.S. Pat. No. 6,434,252 extends the same approach by concentrating the magnetic field in a gap between two pairs of opposing magnets set angularly with respect to the ribbon so as to taper, where the gap is defined by a thin pole strip on each side of the ribbon. Each pole strip connects like magnetic poles, N to N and S to S. The ribbon is tuned by tensioning to obtain a flat frequency response when amplified. Whereas Olson taught a “limp” ribbon having a very weak restorative stiffness and a native resonance frequency of perhaps 10 Hz, Royer teaches a resonant mode at 70-90 Hz. More details of the pickup transformer are described in companion US Pat. Publ. No. 2006/0078152. The microphone is bidirectional, accepting sound from front and rear while rejecting sounds from either side. U.S. Pat. No. 6,434,252 also teaches an offset ribbon for capturing higher amplitude sounds. Disadvantageously, the microphone requires high precision assembly and maintenance.
As detailed in the earliest descriptions (U.S. Pat. No. 1,885,001), the ribbon and apposing magnets create a sound shadow. A sound wave will encircle a solid body, termed here a “baffle”, and can be visualized as spreading ripples on the surface of a pond; with destructive interference where the waves wrap around and collide behind the solid body. This is depicted in FIG. 1A. Thus, dependent on the frequency, the compression wave is split, and on the back of the ribbon may be phase-shifted relative to the corresponding compression wave incident on the front of the ribbon, so that one wave cancels out the other. As the half-wavelength of the wave shortens relative to the fixed path length from front to back of the ribbon, some of the energy of the wave is lost in interference (where compression wavelets on the back of the ribbon oppose the compression wavelet incident on the front) thus limiting the electrical potential that results. This phenomenon was demonstrated by Olson by using baffles of varying width to cap the frequency response range. In a preferred embodiment (perhaps state of the art at the time), Olson crafted a baffle wall from a fence of cylindrical rods, each picket of the fence separated by a gap, the rods being connected at the ends by a narrow pole piece proximate to the ribbon. Olson was able to show a response limit approaching 10 KHz using a 1 inch baffle wall versus a 1 KHz limit using an 8 inch baffle wall. One skilled in the art will recognize that a reduction in the path length around a baffle, pole piece, shoe or magnet member will result directly in an improved response to higher frequencies. Sound above 20 KHz is generally not heard by the human ear, so the goal of full range sound quality is not unreachable and a significant body of work has been directed at achieving acoustic fidelity over a higher frequency range.
However, as sensitivity has increased (due to newer and stronger magnetic materials such as rare earth permanent magnets), there is also an increasing need for improvement in suppression of resonance transfer from the body of the microphone, which may be responsible for some of the peaks noted by Duncan in FIG. 4 of U.S. Pat. No. 2,552,311. This is particularly problematic for the metallic microphone bodies that are used to protect the delicate ribbon, and result in undesirable vibration, rumble, harmonic resonance of the protective body, resonance from higher sound pressure levels (such as from loud musical instruments or speakers), and from external bump-associated extraneous signals. Crowley, in U.S. Pat. No. 7,900,337, depicts a suspension for isolating the microphone body and the ribbon to reduce external bumps and resonance as shown in FIGS. 3 through 5, where elastomeric cords that dampen induced body motion are depicted. The ribbon as taught by Crowley, is uncoupled from the body by these acoustically lossy spacers (col 2, lines 56-65).
Similar teachings are seen for example in US Pat. Publ. No. 2009/0279730 to Sank and are accepted teachings in the art. The prior art teaches that the ribbon transducer and the external housing or frame are best uncoupled from each other—ensuring isolation of the ribbon from sounds originating from or resonating from the frame or housing support. This results in the somewhat ungainly suspension systems as devised by Sank and Crowley and in the widespread use of lossy spacers, including rubber or silicon washers, between the housing body and the ribbon-magnet transducer assembly.
Part of the attractiveness of ribbon microphones is the fidelity of the sound reproduction, but also a characteristic dampening that tempers or “colors” the higher frequencies. It is desirable that any improvement in ribbon microphones preserve or even enhance this quality.
Thus, there is a need in the art, for a ribbon support system that preserves the desirable qualities and overcomes the disadvantages of conventional ribbon microphones, supporting the ribbon while eliminating extraneous noise and internal resonances. Surprisingly, accessory pole pieces or shoes may be entirely eliminated from the magnet design, and by acoustically coupling the ribbon to a lower impedance body material, the rich tonal quality or “color” of ribbon microphones is enhanced, not deadened.